job: save the real exception when unpickling fails

We raise our own exception which hides the real error (often an ImportError),
making it difficult to see what happend. Instead, save the original exception
too.

rq/exceptions.py

@@ -11,6 +11,6 @@ class NoQueueError(Exception):
 
 
 class UnpickleError(Exception):
-    def __init__(self, message, raw_data):
-        super(UnpickleError, self).__init__(message)
+    def __init__(self, message, raw_data, inner_exception):
+        super(UnpickleError, self).__init__(message, inner_exception)
         self.raw_data = raw_data

rq/job.py

@@ -27,8 +27,8 @@ def unpickle(pickled_string):
     """
     try:
         obj = loads(pickled_string)
-    except (StandardError, UnpicklingError):
-        raise UnpickleError('Could not unpickle.', pickled_string)
+    except (StandardError, UnpicklingError), e:
+        raise UnpickleError('Could not unpickle.', pickled_string, e)
     return obj